Two-dimensional ionising particle detector

ABSTRACT

A two-dimensional ionising particle detector including a matrix of detecting fibers, each detecting fiber forming a pixel of the detector. Each detecting fiber is composed of a glass capillary filled with a liquid scintillator for which the chemical composition is chosen such that an average free path of primary scintillation photons is negligible compared with a diameter of the capillary. The detector is applicable, for example, to high resolution particle imagery.

TECHNICAL DOMAIN AND PRIOR ART

The invention relates to a two-dimensional ionising particle detector.

For example, the invention is applicable to the domain of particleimagery with a high penetration capacity.

Particle imagery with a high penetration capacity (for example fastneutrons or gamma rays) requires detectors with a good resolution and ahigh stopping power.

For example, this type of detector is used for the fusion of deuterium(DD) or a mix of deuterium (D) and tritium (T) by inertial confinementusing a power laser. Fusion of these hydrogen isotopes takes place in avolume with a 50 μm characteristic dimension. The fusion nuclearreaction is accompanied by release of a 14.1 MeV fast neutron for a DTmix or a 2.45 MeV fast neutron for a DD mix. Fast neutrons havesufficiently long free path to come out of the fuel. The neutronic imagelocalises the area in which hydrogen isotopes burn. The neutronic imageor the gamma image is formed either by a pinhole, or by a coded aperturesuch as a penumbra diaphragm or a ring. Detectors with a high detectioncapacity and capable of locating the interaction point of the particleare necessary for recording this image.

At the moment, two-dimensional ionising particle detectors are made byassembling thousands of plastic scintillator fibres, the length of eachfibre typically being between 1 and 10 cm and forming one pixel of thedetector. Such a detector is shown in FIGS. 1A and 1B. A set of plasticscintillator fibres 2 is held in a cylinder 1. Each plastic scintillatorfibre 2 has approximately the same diameter D, for example 1 mm.

FIG. 2 shows a plastic scintillator fibre. It is composed of a plasticscintillator bar 3 with a high refraction index (typically of the orderof 1.6) surrounded by a duct 4 with a lower optical index (typically ofthe order of 1.5). Incident particles to be detected P (neutrons, gammaradiation) follow a path parallel to the axis of the fibre and deposittheir energy in the plastic scintillator. Backflow ions I are createdand a fraction of the deposited energy is converted into primary photonsPh1, and then into secondary photons Ph2 and tertiary photons Ph3. Thetertiary photons Ph3 form visible scintillation light that is guided asfar as one end of the fibre where an image is recorded using a CCD(Charge Coupled Device) detector. Several centimeters of fibre arenecessary to efficiently detect highly penetrating particles like fastneutrons.

For fibres longer than one centimeter, this technology limits theminimum fibre diameter to about 0.5 mm.

It is also known that sampling an image limits the final resolution inthe source to twice the size of a pixel divided by magnification of theimagery system. Therefore in the event, magnification of an imagerysystem needs to be of the order of 200 to obtain spatial resolutionsless than the size of the source, for example resolutions of the orderof 5 μm. The measurement instrument then extends over long distancesthat may be more than about ten meters.

Moreover, a detector is made by the painstaking assembly of severalthousand pixels one by one. The result is imperfections in the regulararrangement of pixels. Furthermore, the lack of stiffness of plasticscintillator fibres and their high expansion makes it impossible toguarantee precise colinearity between each fibre.

Furthermore, interaction of fast neutrons in a plastic scintillator isdominated by elastic diffusion on hydrogen. Thus, backflow ions Ideposit their energy on a cylinder with a typical diameter of 1 mm whileincident particles (neutrons, gamma radiation) have an energy of 14.1MeV. Therefore another limitation of the spatial resolution in thesource is the width of energy deposition (cylinder diameter) divided bythe magnification.

Thus, the technology for manufacturing two-dimensional detectorsaccording to known art limits the performances of the instruments inwhich these detectors are located. For example, in a matrix of plasticscintillator fibres with a diameter of 0.5 mm, the spatial resolution ofthe neutron detector is limited to 1.4 mm for 14.1 MeV neutrons and 1 mmfor 2.45 MeV neutrons.

The invention does not have the disadvantages mentioned above.

PRESENTATION OF THE INVENTION

The invention relates to a two-dimensional ionising particle detectorcomprising a matrix of detecting fibres, each detecting fibre forming apixel of the detector and including a scintillator to emit scintillationlight, characterised in that each detecting fibre is composed of a glasscapillary filled with a liquid scintillator for which the chemicalcomposition is chosen such that the average free path of the primaryscintillation photons is negligible compared with the diameter of thecapillary.

Other characteristics and advantages of the invention will become clearafter reading a preferred embodiment with reference to the attachedfigures among which:

FIG. 1A shows a two-dimensional ionising particle detector according toprior art;

FIG. 1B shows a detailed view of FIG. 1A;

FIG. 2 shows the interaction of ionising particles to be detected in aplastic scintillator fibre according to prior art;

FIG. 3 shows a two-dimensional ionising particle detector according to apreferred embodiment of the invention.

The same marks denote the same elements in all figures.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 3 shows a two-dimensional ionising particle detector according tothe invention.

The two-dimensional detector according to the invention comprises acapillary matrix 6 filled with a liquid scintillator. The capillarymatrix 6 is placed in a vat 5. For example, the capillaries have anaverage diameter d less than or equal to 500 μm and can be as small as20 μm. The refraction index of glass in the capillaries may for examplebe 1.49. The parallelism of capillaries is less than 100 micro-radians.The path of incident particles is parallel to the centre line of thecapillaries.

For example, the liquid scintillator has a refraction index of 1.57. Thechemical composition of the liquid scintillator is chosen such that theaverage free path of primary scintillation photons is negligiblecompared with the diameter of the capillary. For example, the wavelengthof primary scintillation photons induced in the solvent will be 300 nm.

The liquid scintillator is either a binary liquid scintillator or aternary liquid scintillator. For a binary liquid scintillator, theliquid scintillator comprises a first scintillator component thatabsorbs primary scintillation UV photons to emit secondary emission witha longer wavelength, for example 370 nm. A ternary scintillator liquid,apart from the first component, comprises a second scintillatorcomponent that absorbs the secondary emission emitted by the firstcomponent to in turn emit at a wavelength of between 400 nm and 500 nm,for example 420 nm. In both cases, the refraction index of the liquidscintillator and the refraction index of the glass that forms thecapillary are chosen to guide scintillation light to an output end ofthe capillary.

The solvent that forms the capillary may for example be PXE (forphenyl-o-xylylethane). As a non-limitative example, the spatialresolution of the binary liquid scintillator is 6 μm and it emits at 370nm and the spatial resolution of the ternary liquid scintillator is 7 μmand it emits at 420 nm. Binary and ternary scintillators may for examplebe components marketed under references EJ-399-05C2 and EJ-399-05C1respectively.

Preferably, the liquid scintillator contains deuterium. The use ofdeuterium can advantageously reduce the width of the neutron energydeposition area about its interaction point, by a factor of 2. Theliquid may also contain a solution of lithium or an element with anatomic mass greater than lithium. Moreover, the intensity of thescintillation emission will be divided by a factor e (e≈2.71828) in afew nano-seconds. This property makes it possible to select the neutronenergy band per flight time. This property also makes it possible todifferentiate neutrons from photons that usually accompany theproduction of neutrons. Due to its nature, the binary scintillator has arise time of a few tens of pico-seconds. This property is essential, forexample, for ultra-fast subnanosecond cinematographic applications.

The vat 5 comprises a first wall 7 fitted with a glass port transparentat the scintillation wavelength and a second wall 8 located in front ofthe first wall and made of a mirror reflecting at this wavelength. Inthe vat, the capillaries are placed between the window and the mirrorand their axis is perpendicular to the mirror and the port. Particles tobe detected penetrate into the detector through the mirror.Scintillation light is collected through the port 7. This light isemitted isotropically, and the fraction of light emitted that goestowards the mirror is reflected by the mirror and is returned to theoutput port.

Elastic membranes 9 and 10 on the top and bottom walls respectively ofthe vat, which are parallel to the axis of the capillaries, absorbthermal expansions of the scintillator.

For example, the detectors matrix has a section of the order of 100×100mm² and a thickness E that may vary from 10 to 50 mm. It is made from asingle block by multiple assembly of macro bundles containing elementarybundles. Large section monolithic detectors can be made with thistechnique. The capillaries matrix is preferably made on a thickness muchgreater than the thickness required, so as to assure good colinearitybetween capillaries (for example less than 100μradians).

A digital embodiment example of a detector used to acquire the neutronicimage of a 1 mm diameter capsule filled with deuterium and imploded by a30 kJ laser is given below. The capillaries matrix is a block with aside dimension of 100 mm and 50 mm thick. Each capillary has a diameterof 250 μm. The liquid scintillator with an optical index of 1.57contains 98% deuterium. Its scintillation efficiency is 80% comparedwith anthracene and its decay constant is 3 ns. The stainless steel vatis closed by a mirror and a glass port. Four elastic membranes enablethermal expansion of the scintillator.

1. A two-dimensional ionising particle detector comprising: a matrix ofdetecting fibers, each detecting fiber forming a pixel of the detectorand including a scintillator to emit scintillation light, wherein eachdetecting fiber comprises a glass capillary filled with a liquidscintillator for which a chemical composition is chosen such that anaverage free path of primary scintillation photons is negligiblecompared with a diameter of the capillary.
 2. A two-dimensional ionisingparticle detector according to claim 1, wherein the capillaries areplaced in a vat comprising a first wall fitted with a glass porttransparent at a wavelength of the scintillation light and a second walllocated in front of the first wall and comprising a mirror reflecting atthe wavelength, ionising particles penetrating into the detector throughthe mirror.
 3. A two-dimensional ionising particle detector according toclaim 2, wherein the vat comprises a top wall and bottom walls thatcomprise elastic membranes to absorb thermal expansion.
 4. Atwo-dimensional ionising particle detector according to claim 1, whereinthe liquid scintillator is a binary liquid scintillator.
 5. Atwo-dimensional ionising particle detector according to claim 1, whereinthe liquid scintillator is a ternary liquid scintillator.
 6. Atwo-dimensional ionising particle detector according to claim 1, whereina solvent used in the liquid scintillator includes PXE.
 7. Atwo-dimensional ionising particle detector according to claim 1, whereinthe liquid scintillator comprises deuterium.
 8. A two-dimensionalionising particle detector according to claim 1, wherein the capillarieshave a diameter between 20 μm and 500 μm and a length between 10 and 50mm, and the matrix has a section approximately equal to 100×100 mm².